The invention relates to an optical device for access to a track on a data carrier and an optical memory system incorporating such a device.
The invention more particularly relates to reader and recorder systems in which the data carrier is a disk. At present, it is possible to record approximately 10.sup.10 data bits on a disk having a diameter of about 30 cm. These systems can be used as a mass memory or digital data processing machines. These systems must permit a random source to a predetermined part of the recorded data, for example a block of binary words of fixed or variable length. In addition, the computing speed of the data processing units requires numerous changes between the peripheral memories and these computing units. It is therefore necessary that access to a predetermined track of a moving data support takes place in the shortest possible time, either for reading prerecorded data or for recording such data, no matter what the position of this track on the disk. The average access time must be below 100 milliseconds.
The presently available track access devices are substantially intended for disks on which the recorded data contains a video signal. In such systems, the data access time is a few seconds, which is sufficient for this purpose.
In the prior art access devices, the correct radial positioning of an optical reading and/or recording head is obtained by mechanical means ensuring either the displacement of the head, or most frequently the displacement of the disk. When the reading head and disk are correctly positioned relative to one another, the radial following of the circular or helical track on which used to be recorded or is already recorded the data is brought about by means of a galvanometric mirror which moves around an axis parallel to the plane of the disks, which reflects a beam produced by at least one light energy source incorporating a laser. The head also has a device ensuring the vertical control of the lens used for recording and/or reading.
No matter what solution is adopted the moving means has too great a mass to be compatible with the average access times desired in data processing. For example, in the solution involving displacing the disk, the latter being fixed to a rotary mechanism particularly incorporating the drive motor, a mass of approximately 1 kg must be set in motion.